The invention relates to a method for cutting to length a strip-shaped profiled member made of unvulcanized rubber mixture by a cutting edge.
Rubber in the unvulcanized state exhibits a substantially plastic deformation behavior under load. This property is taken advantage of when shaping a rubber article. For articles where the shaping, on the one hand, and the vulcanization, on the other hand, are done in the same tool, conventionally called a vulcanization mode, the vulcanization speed is adjusted by the dosage of vulcanization accelerators, and optionally antagonistically acting vulcanization retarding agents, as well as the temperature curve such that during the first seconds up to a few minutes after introduction of the prepared green tire, especially during closing of the mold and optionally inflating of the green tire, the vulcanization is very slow so that the rubber is substantially still in a plastic state. For example, this allows for the highly precise imprinting of the tread pattern into the vehicle tire. Only after completion of imprinting vulcanization continues while the mode is still closed such that the unvulcanized rubber mixture turns into vulcanized rubber, i.e., its plasticity is greatly reduced in favor of increased elasticity. The rebound resilience of modern tire tread mixtures after vulcanization is approximately 70%.
A cutting edge is a sharp edge where two planar or slightly curved blade surfaces having a substantially constant angle, the ground angle, over the entire arc length of the cutting edge, meet. A knife is a tool for separating which includes a blade plus at least one further component having a different function, for example, a grip, a fasting device, a motor, etc.
For building respective green tires, diverse strip-shaped unvulcanized profiled members must be cut to length, for example, tread strips. Especially for unreinforced rubber strips, i.e. strips that do not contain reinforcement elements such as cords or cables, the problem that the cutting reaction force of the material to be cut, which is oppositely oriented to the cutting blade advancing force of the same amount, results in a permanent deformation of the material to be cut at both sides of the produced cut, which during the further production process makes joining of the different components more difficult and impairs the precision of the final product.
It is also well known that, when using a disc-shaped cutting edge, which is rotated at high speed, instead of a cutting edge which is to be moved only translatorily, the cutting reaction force can be lowered. For sufficiently small advancing speeds in relation to the circumferential speed (=2.times.pi.times.rpm of the knife) surprisingly clean cuts result. However, it has been observed that contaminants at the top surface are conveyed by the blade of the knife onto the freshly produced, and thus especially tacky, end face of the strip where they will remain. This impedes adhesive strength at the finished product. Furthermore, cutting devices with a rotating blade are rather expensive and prone to malfunction.
It is an object of the present invention to provide a method for cutting to length which no longer has the aforementioned disadvantages. Especially, the permanent deformation of the material to be cut to length resulting from the performed cut must be negligible and the contaminant transport from the top surface of the profiled member to be cut onto the cut end face must be prevented.
Furthermore, an especially reliable function during constant use in mass production of rubber products is desired, especially in the manufacture of pneumatic vehicle tires, especially for cutting to length the tread strip.